1. Technical Field
The present invention relates to a lens device which is suitable for broadcast or a movie and a position detection method of a movable optical element which is mounted in the lens device.
2. Related Art
Recently, as a large screen size and high definition of a television or a monitor has progressed, a demand on a high quality of an image to be projected is increased. In order to satisfy the demand on the high quality image, a position detector which detects a position with high precision is mounted in a zoom lens for a movie or broadcast to increase the performance of lens control.
While not for a lens device, a position detector is disclosed in Patent Literature 1 (JP-A-6-58766), Patent Literature 2 (JP-A-2002-250639) and Patent Literature 3 (JP-A-2004-507722).
A position detector disclosed in Patent Literature 1 includes two annular magnetic recording media which are recorded with magnetic signals having different wavelengths and two MR sensors which detect signals corresponding to the magnetic signals of the two annular magnetic recording media, respectively. Further, the position detector calculates a phase difference between sine waves which are respectively output from the MR sensors, based on sine waves and cosine waves which are respectively output from the two MR sensors, and detects an absolute position of the annular magnetic recording medium based on the phase difference.
A position detector disclosed in Patent Literature 2 includes two magnetic recording media which are recorded with magnetic signals having different wavelengths and two MR sensors which detect signals corresponding to the magnetic signals of the two magnetic recording media, respectively. Further, the position detector calculates a phase difference between sine waves which are respectively output from the MR sensors, based on sine waves and cosine waves which are respectively output from the two MR sensors, and obtains an initial absolute position of the magnetic recording medium based on the phase difference. Thereafter, the position detector compares a temporarily detected position of the magnetic recording medium which is obtained by incrementing the initial absolute position and an absolute position which is obtained based on the phase difference to calculate a true absolute position based on the comparison.
A position detector disclosed in Patent Literature 3 includes two annular magnetic recording media which are recorded with magnetic signals having different wavelengths and a plurality of MR sensors which is provided for each of the two annular magnetic recording media. The position detector calculates averages of sine waves and cosine waves which are respectively output from the plurality of MR sensors and calculates a phase difference between the sine waves which are respectively output from the annular magnetic recording media, based on the average of the sine waves and the average of the cosine waves which are calculated for each of the annular magnetic recording media.